Clang can build a kernel now. "The kernel can successfully boot to runlevel 5 (aka X + networking) on the Macbook, both on bare metal and in Qemu. The kernel can successfully boot to runlevel 3 on a secondary test machine, a microATX desktop box (Intel Atom). I haven't tried to start X on this box yet. The kernel can self-host; I am currently running a 'fourth generation' self-hosted Linux kernel built by a 'fourth generation' Clang."

clang and llvm have been imported into the FreeBSD source tree, and it will be available as part of FreeBSD 9.0. It can build the entire FreeBSD OS (kernel, libs, boot loader, userland, toolchain), and can rebuild it all using the resulting system (ie, self-hosting). You can even build/run a FreeBSD system without GCC installed at all.

It's not currently possible to replace GCC for building apps via the ports tree. There are quite a few apps that fail if CC != cc (gcc). But, as these are reported, fixes are sent upstream, and (hopefully) things get better in the long-run. For those that don't build with clang, a version of GCC can be automatically installed via the ports tree, and that can be used to build the app.

Long-term, it's very likely that GCC will disappear from the FreeBSD base install and source tree.